1. Field of the Invention
The invention relates to a method of preparing layers of vinylidene fluoride polymers and vinylidene fluoride/trifluoroethylene copolymers.
2. Description of Related Art
For applications in microelectronic engineering, for example as integratable single layer capacitors (with a capacitance density &gt;10 nF/cm.sup.2) for storage modules, or as thin layer piezo- or pyroelectric elements, polymers are required on a plastic base with ferroelectric properties or at least with a dielectric constant &gt;5 in an unpolarized state. The only known representatives of ferroelectric plastics are polymers consisting of vinylidene fluoride, i.e. polyvinylidene fluoride (PVDF), and copolymers consisting of vinylidene fluoride and trifluoroethylene (c.f. e.g.: "Appl. Phys. Lett.", Vol. 36 (1980), pp 286 to 288).
Heretofore, the synthesis of pqlyvinylidene fluoride, i.e. poly(1,1-difluoroethylene), and vinylidene fluoride/trifluoroethylene copolymers has taken place exclusively in a conventional chemical manner; that is, by means of polymerization of the corresponding monomers with the aid of catalyzers, e.g. through peroxide catalyzed emulsion- or suspension (bead) polymerization of vinylidene fluoride at an elevated temperature and an elevated pressure. The polymers or copolymers are then processed into cast films, e.g. from solutions in methyl-ethyl-ketone. Next, the cast films are stretched at temperatures above 100.degree. C. to four times the original length to attain ferroelectric properties, and in this state they are frozen by means of rapid cooling and/or are polarized by means of electric fields up to 100 U/.mu.m.
Presently, the known ferroelectric plastics are only available as foils with a minimum thickness of approximately 10 .mu.m. For applications in microelectronic engineering, however, these materials would have to have a much smaller thickness, indeed down to the sub-.mu.m-range.
It is an object of the present invention to specify a method which permits the preparation of layers consisting of vinylidene fluoride polymers and vinylidene fluoride/trifluoroethylene copolymers having a thickness of &gt;10 .mu.m, where the dielectric constant or the ferroelectric properties of the layers correspond to those of the presently known materials.